Methods for detecting combustion misfires are utilized in spark-injection engines in order, on the one hand, to detect an uneven running of the engine and to minimize this uneven running by suitable control arrangements while, on the other hand, most of all, to avoid a deterioration of the exhaust-gas values caused by misfires and to protect the exhaust-gas catalytic converter. The uneven running of the engine can be caused, for example, by valve coking or by quality scattering of characteristic values of the injection valves for direct injection.
Methods of this kind utilize the recognition that a non-occurring combustion within a cylinder of an engine is associated with characteristic changes of the torque trace of the engine compared to the normal operation. One can distinguish between normal operation of the engine without misfires and an operation with misfires via the comparison of the torque traces. An operation with misfires in one or several cylinders contributes to the total torque course of the engine with a lesser contribution and this contribution can be determined from a detection of the actual torques of the cylinders via an evaluation of the time-dependent trace of the crankshaft rotation or camshaft rotation.
In a known method, a specific region of the piston movement of each cylinder is assigned a crankshaft angle region identified as a segment. The segments, which belong to each cylinder are, for example, realized by markings on a transducer wheel coupled to the crankshaft. The segment time, that is, the time in which the crankshaft passes through the corresponding angular region of the segment, is essentially dependent upon the energy converted in the combustion stroke. Misfires lead to an increase of the ignition-synchronously detected segment times as a consequence of the insufficient torque contribution. These detected segment times are determined for each cylinder by scanning the markings on the transducer wheel via a suitable sensor. The more even the engine runs, the lesser are the differences between the segment times of the individual cylinders.
A method is disclosed, for example, in German patent publication 4,138,765 (corresponding to U.S. patent application Ser. No. 07/818,884, filed Jan. 10, 1992, now abandoned) wherein an index for the rough running of the engine is computed from differences of the segment times and wherein general conditions, for example, the increase of the engine rpm in a vehicle acceleration, are compensated by computation. The rough-running value which is computed in this manner for each ignition is, in a next method step, ignition-synchronously compared to a threshold value in a desired value comparison. If the determined rough-running value exceeds the threshold value, which is dependent, if required, on operating parameters such as load and rpm, then this is evaluated as a misfire of the particular cylinder. If misfires are determined, a suitable warning device can provide an indication of this irregularity when a certain misfire rate is exceeded.
Alternatively, and in a further method step, a cylinder equalization can take place after detection of the misfire operation. For this purpose, equalization or corrective factors are formed individually for each cylinder in an evaluation unit and, with the aid of these factors, injection times or ignition time points of the individual cylinders, which are affected by the misfires, can be influenced. Thus, a change of the ignition time point can, for example, eliminate the incomplete combustion of the gas mixture within a cylinder so that this cylinder again can provide the full contribution to the total torque of the engine. Furthermore, the differences in the injection performance of injection valves can be compensated by influencing the injection times and the injection duration.
From the foregoing, it becomes clear that a reliable detection of engine misfires contributes to compliance with statutory provisions and the detection of engine defects. The detection is dependent, inter alia, essentially on the desired value comparison of the determined rough-running values with the pregiven threshold values. Depending upon the setting of this threshold value, a reliable detection of misfires takes place or individual misfires are not detected. Accordingly, the setting of the threshold values for the determined rough-running values by the engine manufacturer is of great significance because, only by recognizing misfire operation, can a corresponding adaptation of injection times and ignition time points take place and the rough-running values can be reduced or the vehicle user can be made aware of an engine defect.
The method of the invention for detecting combustion misfires in multi-cylinder engines affords the advantage with respect to the known state of the art that the detection quality of combustion misfires is significantly improved so that a considerably better assurance is provided as to erroneous detections of misfires, that is, an assurance is provided against normal combustions being evaluated as combustion misfires. This advantage is achieved in that the determined rough-running values LUT or filtered rough-running values FLUT are compared in a desired value comparison to a threshold value SW2, which is pregiven in value considerably less than the threshold value SW1 for detecting misfires. A reduction of the threshold value SW1 for detecting misfires to a lower level results when there is a persistent drop below the second threshold value SW2 within a pregiven time interval xcex94t or during a specific number of engine revolutions xcex94u.
With this feature of the invention, the equalization of cylinders (that is, the smooth running of the engine) is utilized to increase the disturbance intervals of the misfire detection. More specifically, the equalization is achieved as a consequence of influencing the combustion operations in individual cylinders by changing injection times and ignition time points for these cylinders. The relevant threshold value, which is applied to detect misfires, can be adjusted to be so sensitive that as many misfires as possible are detected. This sensitivity is achieved because of the smaller rough-running values which are determined over a longer time span. Accordingly, for a relatively good smooth running of the engine (that is, at low rough-running values), the threshold value does not have to be insensitive as is usual in the state of the art which causes a poor detection quality of the combustion misfires to result.
In connection with the change of the threshold value necessary to detect combustion misfires, it has been shown to be especially practical to set this threshold value to the value of an available lower threshold value stored in the control system. This affords advantages especially for programming the electronic evaluation circuits.
Furthermore, it has been shown to be advantageous that exceeding the threshold value SW2 effects a raising of the threshold value SW1. Notwithstanding a continuously executed equalization of cylinders, it cannot be precluded that, for special reasons, individual cylinders temporarily provide a changed contribution to the overall torque of the engine. For this reason, an erroneous estimate of combustion misfires is avoided by the measures described. The resetting effects essentially a reestablishment of the method parameters to the start of the xe2x80x9clearn effectxe2x80x9d of the method of the invention and leads to the situation that a reduction of the threshold value for combustion misfire detection can take place when the cylinder equalization again brings about an increased smooth running of the engine. This is achieved with a renewed comparison of the rough-running values to the lower threshold value with the comparison being carried out over a certain time span.
The threshold value is purposefully reset to the original value established at the start of the method.
For specific areas of application, it can be advantageous to carry out the reduction of the threshold value for combustion misfire detection to a lower level in several stages. The individual threshold values lie between the magnitude of the threshold value for combustion misfire detection and the reduced threshold value. In this way, a slow approach of the combustion misfire detection can take place to an especially low threshold value level and, at the same time, the problem is avoided of evaluating an essentially proper combustion as a combustion misfire because the corresponding threshold value, which is applied as a desired value comparison, is to be viewed as being too sensitive.
For the practical realization of the method as well as a compact configuration of the necessary components, it can be advantageous to store the equalization or corrective factors after each desired/actual comparison of the rough-running values in a precontrol characteristic field and, after reducing the first threshold value to a lower level, the last stored equalization or corrective factors are left unchanged. The equalization or corrective factors are determined for each cylinder individually from the rough-running values. In this way, the precontrol characteristic field contains the equalization or corrective factors, which are necessary for optimal equalization of the individual cylinders, only after a corresponding learn phase, that is, after a time interval in which the rough-running values are at so low a level that, in principle, a misfire-free operation can be assumed.